Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 92
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 92, with English translation by Maimonides (Rambam).
לגדל הנזיר שיערו, שנאמר: "גדל פרע, שיער ראשו" (במדבר ו,ה).
For a Nazirite to grow his hair long, as [Numbers 6:5] states: "He shall let the hair of his head grow without cutting it."